Severe Weather
- ~75% of yearly deaths and damages from natural disasters
- More people are usually killed by severe weather than EQs, volcanoes, mass movements combined
- Frequency of $1bil+ events is increasing
Winter Storms: Cold
Hypothermia: begins when body temp drops below 35C, deadly
- Most hypothermia deaths are associated with outdoor rec or disabled vehicles today
- Wind exacerbates cold temp by stealing heat from body
Winter Storms: Precipitation
Precipitation as snowflakes or ice particles/hail
- When snow or ice fall through warmer air, they may melt and continue as rain or enter below-freezing air where it:
~ refreezes into tiny ice particles = sleet
~ supercools to freezing rain and freezes upon impact with subfreezing surfaces
- Falling snow also accumulates on snow
Blizzards
- Strong cold winds >56kph and blowing/falling snow reduce visibilities to <400m
- Cyclone may travel slowly even though winds are fast
Ice Storms
Large volume of supercooled rain that freezes on impact (freezing rain)
- Adds mass to trees, power lines, roofs = collapse
Thunderstorms
Tall, buoyant clouds of rising moist air that generate lightning and thunder, often with rain, wind, and sometimes hail
- Air temp decreases with alt = unstable conditions
- Rising warm, moist air may begin condensing = cumulus clouds and releasing latent heat = energy needed for severe weather
- If warm air continues to rise, cumulonimbus clouds can build up to 20km high
Thunderstorms develop from airlifted by 3 different mechanisms:
1) Convectional lifting
2) Frontal lifting
3) Orographic lifting
Convectional lifting
Surface heated air rises buoyantly since it’s less dense, clouds form locally
Frontal lifting
Air masses collide at frontal boundaries, warmer air mass rises to form clouds
Orographic lifting
Air mass flows up a steep slope, expands and cools = increases relative humidity to form clouds and thunderstorms
Air-mass Thunderstorms
- Common in afternoons/evenings
- Year round in tropics, summer in midlats
- Rising, moist warm mass cools, condenses into cumulus clouds, continued updraft yields cumulonimbus
- Heavy rain produces downdraft
- Warm updraft and cool downdraft side-by-side
- Downdraft and rain evaporation (absorbs heat) at surface extinguishes updraft then storm ends
Severe Thunderstorms
Severe when: winds >93kph and hail diameter >25mm
- Commonly form at long (100-1000km) frontal collisions, allow up- and down- drafts @ same time
- Much smaller cyclonic thunderstorms with high wind speeds occur within larger systems
Thunderstorms in Canada
- Distribution of lightening activity is non-uniform pattern, Southern Ontario has the most thunderstorms
Severe Thunderstorms: Supercells
Supercell thunderstorms: violent severe thunderstorms with huge updrafts
Really moist and warm air rises than hits troposphere where temp continues to increase = massive cloud
- 20-50km rotating mass or mesocyclone
- Vortex is rotating updraft about vertical axis
- Rain and hail fall at leading edge
- Potential powerful tornadoes spin of trailing edge
Thunderstorms in North America
- Not geographically homogenous, most common in Florida, where Atlantic and Gulf of Mexico warm, moist air masses meet
- Common in central and southern US states (warm moist air from Mexico hits cold air masses moving from north)
Heavy Rains and Flash Floods
Thunderstorms can be a major supplier of water to an area = flash floods
Hail
Layered ice balls dropped from storms with:
- Buoyantly rising, hot, moist air -> keep moisture aloft in circular motion and accumulates water which freezers, then melts, then freezes again = layered
- Upper-lvl cold air creates large temp contrasts
- Strong updrafts keep hailstones aloft while adding layers
Most common in late spring and summer, along jetstream in colder midcontinent
Tornadoes
Rapidly rotating column of air from large thunderstorm, spawn out of leading edge of supercells
- Highest wind speeds of any weather phenomenon
- 70% of Earth’s tornadoes occur in Tornado Alley in US, move from SW to NE
- Travel up to 100kph, wind speed can be >500kph
- Low pressure in core of vortex (<1km wide) and high velocity air = sucks up objects
- Commonly form high in atmosphere, many never touch ground or travel many km in irregular path along ground
Tornadoes in 2011
- 2nd deadliest tornado year with 553 deaths
- 1706 confirmed tornadoes in US
- Six EF5 (max cat) events with winds in access of 200mph
- 24 tornado deaths everywhere else in world
Regional Scale Formation of Tornadoes
3 air masses moving in different directions meet and give shear to thunderstorm
- Wind, humid, low Gulf of Mexico air + cold, dry, mid-alt Canadian or Rocky Mountain air + fast, high-alt jetstream winds
Rising Gulf air is spun 1 way by mid-alt cold air then spun other way by jetstream = corkscrew effect
- Warm air rising on leading side (rly fast)
- Cold air descending on trailing side
Supercell Thunderstorm Scale Formation of Tornadoes
Tilt of supercell moves warm updraft to center of the storm = rotation that spins off
- Formed below main mass of mesocyclone where updraft is strongest, wall clouds are where powerful tornadoes can emerge
- Rotation develops in wide zone, core pulls into tighter spiral -> speed increases
- Wind speeds highest few hundred m above ground (slowed by friction @ ground)
- Hook echo: high reflectivity area in radar image shaped like a hook, indicator of tornado potential
Vortex Scale Formation of Tornadoes
- Air mass collisions low in atmosphere creates horizontal tube of air spinning parallel to ground
- Warm rising air provides vertical wind shear that lifts spinning tube of air
- As it rises, it stretches, diameter decreases, velocity increases (figure skater)
- Can rise to mesocyclone status
F scale
Wind damage scale (F0-F5) estimating wind speed from damage to structures, trees
- New EF scale is more precise
- Deaths and damages increase from F0-F5, most destruction from minority of large tornadoes
Deaths from:
- Wind blowing away buildings/trees
-Wind-blown debris impacts - Winds remove roofs, lift out contents
